A bone-density test helps determine if you have osteoporosis, a disease that causes bones to become more fragile and more likely to break. In the past, osteoporosis could be detected only after you broke a bone. By that time, however, your bones could be quite weak. A bone density test enhances the accuracy of calculating your risk of breaking bones. Risk depends on gender, age and other health factors. Screening guidelines are available to help determine when to test.
The test uses X-rays to measure how many grams of calcium and other minerals are packed into a segment of bone. The bones that are most commonly tested are in the spine, hip and forearm. Ask your doctor about whether you should be screened for osteoporosis.
Source: Mayo Clinic